You could use a queued process for this reason. I could imagine to create a microflow which starts and creates queued actions, which doing one of your download action, and then create for every dowload a queued action. Every queued action is one microflow which could contatin sub microflows. In the Queue, you can decide then how many of them running in parallel. With that solution, you could run multiple microflows at the same time. If youo search in the App Store, there are two possible Queue modules.